Complete revamp of SQL generation
Fix errors when loading from clean state
Exporting stuff for use by bigrquery
Document and export is.source
Document and export var_eval
Remove outdated version of escape_sql
Add is.null
Document and export dim_desc and trunc_mat
Fix documentation buglet
Better format tag for *_sql
Fix bug in wrap function
Make source_df and grouped_df subclasses of data frame.
Fixes #12
Remove op class